It looks like to me you have 2 issues, extension cord and running too many electrical appliances. On a 30 amp circuit, don't run any heavy amp appliances when using your A/C, especially the water heater. The extension cord is also a problem, you have a 25' cord from your coach then you've added another 50' of extension cord so you are running 75' of extension cord, with the amp drop over this length you don't have 30 amps at your coach. Another potential problem could be the circuit breaker at the post. It may be weak from popping so many times. When 50 amp is available, I always use a 50/30 adapter.

Line in has been 117-124V. Pretty steady around 118-120. No line errors.

Land of 10,000 Lakes
2016 Forester MBS 2401R

These are good voltage numbers, but are they taken under load? The meaningful number is the voltage under load. A long, or under sized extension cord will provide good readings with no load but the added resistance under load will pull the voltage down, and the amps up.

Just to add to this....my 5'er is 30 amp service. I've fought power issues during the heat of summer as well. With 30 amp service, you certainly have to manage your power usage. I bought a 30ft open ended 50 amp power cord and put a 30 amp female plug w/weather boot on it to mate to the trailer. Since then, I've also started running both the frig and water htr on propane. At this point, I think I've conquered the 30 amp blues.

These are good voltage numbers, but are they taken under load? The meaningful number is the voltage under load. A long, or under sized extension cord will provide good readings with no load but the added resistance under load will pull the voltage down, and the amps up.

Thanks all for the help and suggestions. I'm pretty sure it was the extension cord plus the combination of things that were on together.

Later in the trip we needed the heat pump at night and even with extension cords either 50' or 100' in total line (yup) I had no problem with it as long as I turned off the hot water heater. Refrig on but not working too hard in cold nights (down to 45) and converter on.

Would a hard start capacitor on the AC help reduce the amp draw from the AC on start up and therefore the voltage drop? I've had the low voltage problem at a local Provincial Park. The pedestal only had about 110 to 111 volts.